Zinnias are the workhorse of the cutting garden: fast to flower, generous with blooms, and almost impossible to kill in full sun. Sow directly into warm soil after last frost and you are cutting in eight weeks. The real trick is succession: sow every two or three weeks through midsummer and there are still armfuls when the first frosts arrive. They come in every colour but blue, from 12-inch bedding types to 3-foot dahlia-flowered cutting strains like the Benary's Giants.
Space them 9 inches apart in average soil that drains, and water at the base, since wet leaves bring on powdery mildew by late summer. Pinch the first flower bud to force branching. Every stem you cut, taken all the way back to a leaf node, encourages two more. Zinnias stop producing if you let them go to seed, so deadhead ruthlessly and they flower until frost.
Cut when the flower is fully open and the stem is stiff; shake it, and if the head wobbles it is too young and will never open further. Butterflies work the open centres all summer and goldfinches take the seed of any heads you leave. Native to Mexico, zinnias are annuals everywhere in the US. Deer rarely bother them; Japanese beetles sometimes do.
Powdery mildew
White or grey powdery coating on leaves - usually starting on older growth in humid conditions or when nights cool.
Improve air circulation by thinning plants. Apply neem oil or potassium bicarbonate spray at first sign. Avoid overhead watering.
Alternaria blight
Brown circular spots with yellow halos on leaves; spreads rapidly in wet weather.
Remove affected leaves immediately. Water at the base only. A copper-based fungicide can help if caught early.
The white bloom on the leaves
The zinnias are wearing a grey-white dust that wipes off under a finger, and the instinct to reach for a spray is almost always the wrong one to reach for first.
Fasciation
The strange flattened stems appearing in your cutting bed are not a disease - they are one of botany's more beautiful accidents.